When I added another cat, I added another litterbox to the house, in different rooms. My big cat, Murf, used one box for each function, and always seemed to remember which was which.

When I moved to a much smaller place, I didn't have separate rooms for the litter. So, I went to Home Depot (as if I needed a reason) and bought a plastic bin that is used to mix concrete. It has low sides, and is big enough to hold both litterboxes.

The extra inch or so around both boxes helps collect the scattered litter that Murf loves to fling. He's SO enthusiastic about his covering duties. The only drawback is that the bin is black plastic - not a very lovely addition to the decor. Oh well, the cats are happy!